Officers said the man who stopped the sex attack walked the victim away from the scene and the police were called but he left without giving any details.

Police want to speak to the man as a potential witness to the assault.

Det Ch Insp Ian Scott said: "The victim has been left understandably traumatised as a result of this attack and we urgently need to identify the man responsible and arrest him.

"We are carrying out extensive inquiries into the incident and would like to hear from anyone who was in the area and saw a man fitting his description or who saw anything that could assist the investigation.

"We also need to speak to the man who went to the victim's aid as he is an important witness.